Investment Binder and Use of the Investment Binder

ABSTRACT

Investment binder for forming a binder phase when applied to an investment casting, wherein this binder comprises
         I at least a first compound as a source of magnesium-ions,   II a second compound as a source of phosphate-ions and   III a third compound being a surface reaction control agent incorporated in one of the other compounds above, and   use of this investment binder.

FIELD OF THE PRESENT INVENTION

The present invention relates, generally, to an investment binder and the use of the investment binder.

BACKGROUND OF THE PRESENT INVENTION

The use of phosphate-bonded investments in dental or jewellery applications is known in the art. These investments are used to provide a rigid ceramic base in which a molten metal alloy or a precious metal can be cast or a glass ceramic can be pressed. Interaction at the interface of the ceramic investment and cast metal, metal alloy or pressed glass ceramic can lead to formation of a reaction layer. Therefore the cast objects are conventionally treated by an aggressive surface polishing treatment. This treatment may be done by the use of acid for etching with hydrofluoric acid and/or by air blasting using aggressive and abrasive articles, e.g. sand-blasting.

So far magnesium phosphate-bonded investments are well known in dental applications and they form, after their application, various reaction layers dependent upon interaction between the investment and cast metal, alloy, or pressed glass ceramic. General description

The present invention seeks to provide an investment binder that can be used without any further treatment with hydrofluoric acid or sandblasting.

The present invention provides a solution to the aforementioned problems with an investment binder having the features of claim 1.

According to the invention, an investment binder for forming a binder phase when applied to an investment casting comprises

I at least a first compound as a source of magnesium-ions,

II a second compound as a source of phosphate-ions and

III a third compound being a reaction surface control agent incorporated in one of the first or second compounds.

Aggressive sandblasting or acidic etching with hydrofluoric acid or similar aggressive treatments are not needed when such an investment binder is used, because the contact surface is protected by the introduced reaction surface control agent.

Preferred embodiments of the current invention are the subject-matter of the sub claims.

The reaction surface control agent can be selected from a group consisting of hexagonal boron nitride, talc, ion-exchange agents, zeolites, and/or clay-containing minerals including bentonites.

The second compound is preferably an alkali-metal phosphate or metal-phosphate or an ammonium phosphate. These compounds have a good solubility in polar solvents, especially in water or aqueous solutions such as silica sol.

The investment binder can be used for the formation of a dental investment casting and other investment casting or forming techniques.

When used, the investment forms an interface layer between the base material and a glass material and/or metal alloy and/or a precious metal.

According to the invention a process for forming a product includes the following steps:

i. Providing a wax impression of the intended final product;

ii. Providing a ceramic investment material, wherein this providing consists of

-   -   ii-a Providing a slurry, especially an investment binder         mentioned above, which comprises at least a solvent,         magnesium-ions, phosphate-ions and at least one reaction surface         control agent;     -   ii-b Providing a ceramic base material;     -   ii-c Applying the solution of step ii-a to the ceramic base         material of step ii-b;

iii. Casting of the ceramic material around the wax impression by forming a mould

iv. Dewaxing of the mould and applying a glass material, a metal alloy and/or a precious metal to the mould, wherein a binding phase which comprises magnesium phosphate hydrate and the surface control agent forms a ceramic to glass and/or a ceramic to metal interface layer.

The slurry in step ii-a is obtained by co-milling a source of phosphate-ions with the reaction surface control agent and, after this mixing, adding a solvent and a source of magnesium ions. By providing the solution in this way, the concentration of the surface reaction control agent is more homogeneous compared to an addition of this agent after a mixing of magnesium oxide and ammonium phosphate. In addition, the invention provides for a more economical use of expensive surface reaction control agents.

It is advantageous if the ceramic to glass and/or the ceramic to metal interface layer has a homogeneous distribution of surface reaction control agent particles.

DETAILED DESCRIPTION

The invention is explained in more detail by a preferred embodiment. The scope of the current invention is not limited to this example. The present invention relates in particular to control of a cast metal, metal alloy, or pressed glass ceramic surface used, for example, in dental restorations

Investment casting is a technique that belongs to the general common knowledge of someone skilled in the art. A ceramic is cast around a wax impression of the intended final article. After de-waxing, the ceramic becomes a mould into which a molten metal alloy or a metal may be poured.

Alternatively, a glass ceramic can be pressed into the void left by the wax. This can be done in a pressable ceramic oven.

The use of phosphate-bonded investments in dental or jewellery applications is known in the art. These investments are used to provide a rigid ceramic base in which a molten metal alloy or a precious metal can be cast or a glass ceramic can be pressed. Interaction at the interface of the ceramic investment and cast metal, metal alloy, or pressed glass ceramic can lead to formation of a reaction layer. Therefore the investments were treated by an aggressive surface polishing treatment. This treatment may be done by the use of acid for etching with hydrofluoric acid, and by air blasting using aggressive and abrasive articles, e.g. sand-blasting.

In the current invention the ceramic investment is provided with a binding phase forming a matrix to binder phase and includes a surface reaction control agent which inhibits interaction between the ceramic investment and cast metal, metal alloy or pressed ceramic glass.

The following process describes the formation of this binding phase.

In a first step, a ceramic is cast on a wax impression. This ceramic has usually a quartz and/or cristobalite matrix, but not exclusively as the matrix may be selected to provide additional properties, e.g. fused silica for low thermal expansion and mullite for high thermal shock resistance. This forms a base material.

In a second step, a binding agent is at least partially dissolved in a liquid. This liquid could be, e.g. water, or a mixture of water and colloidal silica.

A first compound of this binding agent is preferably a phosphate, most preferably an alkali-metal, metal phosphate, or ammonium-phosphate.

The second compound is a powder of magnesium oxide of varying reactivity dependent upon the properties required. This compound will have low solubility.

During or before the dissolution and mixing of the aforementioned first and second compounds of the binder, at least one of these binder compounds is treated with a surface reaction control agent.

In a preferred embodiment of a process for the formation of a binding phase, the addition of the surface reaction control agent can be done before the compounds are added to the liquid and mixed. In this case the binding agent is dissolved in the liquid and the surface reaction control agent remains in place to prevent the reaction layer from forming.

A preferred surface reaction control agent is preferably selected from a group consisting of hexagonal boron nitride, ion-exchange agents, zeolites, and/or clay-containing minerals including bentonites.

Alternatively, but less preferred, the surface reaction control agent can be added to the mixture of the at least two binder compounds in the liquid. Adding the additives during preparation of the investment can lead to waste of expensive surface reaction control agents without high intensity mixing taking place. This high intensity mixing may well change the reactivity of one or more components of the binding phase by reduction of the particle size distribution, for example of the magnesium oxide, by attrition of these softer elements by the more abrasive matrix.

This addition of the surface reaction control agent is done before the binder is added to the ceramic investment. This ensures that the surface reaction control agent is physically adhered to the surface of the binder component.

The binder reaction of a phosphate-bonded investment can be described as following:

magnesium oxide+X phosphate+water→X magnesium phosphate hydrate,

Wherein X may be a metal, an alkali metal, but more commonly ammonium.

The magnesium phosphate forms a binding phase. This binding phase is formed as a bond between the particles of the investment matrix.

In a third step, a glass compound containing such components as feldspar, lithium silicate, or lithium disilicate, or a metal or alloy such as gold may be applied on the ceramic, wherein the binding layer and matrix on the surface of the ceramic investment forms an interface with the cast metal, alloy, or pressed ceramic glass.

The binding phase does not only form an interface to a glass and/or metal, but also between the quartz and cristobalite or other matrix of the ceramic investment.

Most preferably but not exclusively, the agent should be attached to the phosphate component before it is mixed together with the magnesium oxide in the liquid. When the phosphate ultimately dissolves, the agent remains in place at the surface of the wax impression where it can prevent attack of the metal or ceramic glass when cast or pressed.

The source of magnesium-ions could be a magnesium oxide, preferably with particles having a mean particle size D50 in the range of 3 to 25 microns.

The hexagonal boron nitride can preferably consist of particles having from 50-150%, preferably 80-120%, of the mean particle size of the second component.

A binding to the magnesium oxide is also possible but less preferable than the binding to the phosphate-compound, because of steric effects that may have an influence on the surface and on the control of the binding process when magnesium phosphate is formed. In this case, expensive surface reaction control agents are not used to full economy.

The main advantage of this invention is that the agent is placed exactly where required at the surface of the impression left by the wax. This allows for normal mixing methods to be followed, thereby ensuring control of investment final properties, and the heterogeneity of the surface reaction control agents follows that of the binder through the investment powder.

High intensity mixing of fragile particles such as magnesium oxide or phosphate in an abrasive matrix will lead to a change in the particle size distribution of the binding agent, leading, in turn, to variable properties of the finished investment. This could, in turn, lead to instability of the finished and packaged investment over time, and is a disadvantage of the current state of the art with its addition of such small quantities of powder whilst ensuring homogeneity.

The reduction of reaction layers prevalent in state of the art casting metal, metal alloy and pressable glass ceramics, and caused by interaction of the ceramic investment with metal, metal alloy and glass ceramic, is ensured by rapid mould release, surface protection, and reduction in chemical ion exchange at the investment interface. Glass ceramics, when molten, are tacky and adhere to the surface of the investment mould leading to a reaction layer.

A reaction layer according to the current invention is defined as a solid state interface between a pressed glass ceramic and the dental investment. The layer may be formed from picked up physically scavenged particles of the investment by molten glass under pressure. Preferably, but not exclusively, an ion exchange reaction can take place, particularly between the phosphate and magnesia elements in the binder phase of the investment. This ion exchange reaction could lead to a hard encrusted layer, possibly comprising a compound selected from a group of repolymerised silica lithium phosphate, magnesium silicate, lithium phosphate and/or entrapped investment material attached to the glass ceramic.

The reduced and therefore thinner layer according to the invention is achieved by the mechanism by which the surface reaction control agent is placed in the binding phase. The surface control agent provides the function of a mould release agent and an inhibitor to the suspected ion exchange reaction that forms between the binding phase and the matrix of the mould causing the so called reaction layer.

The need for subsequent aggressive hydrofluoric acid treatment to eradicate this layer is removed, and the strength of the ceramic glass article is unaffected. This is particularly, but not solely, important within the range of lithium silicate and disilicate glass ceramics, where reduced tensile strength and enhanced surface roughness is dramatically affected by acid etching treatment (Braz Dent J (2011) 22(1): 45-50 J Prosthodont. 2008 Jul; 17(5):415-9. Epub 2008 May 9).

The properties of the phosphate, the surface of which comprises a surface reaction control agent, may now be matched with that of a suitable magnesium oxide to ensure all required properties of an investment are within range. Such properties include working time of the suspension as well as setting expansion and compressive strength of the finished and hard set ceramic muffle. The binding phases comprising magnesium oxide and phosphate may be varied at will to control the above properties as in the current state of the art. These properties are not affected by the presence of the surface reaction control agent adhering to the surface of the binder component. Preferentially, the surface reaction control agent should be attached to the surface of the more soluble phosphate phase.

The method of introducing the surface reaction control agent can be described as an in-situ release, because it is released by adding phosphate, which may be coated with the aforementioned agent. This is the most effective way to ensure that this is a homogenous distribution at the low levels concerned. Experimental evidence has shown that manual addition of surface control agents at the same levels does not afford a significant and efficient improvement of reaction layer and at best would result in inconsistent results seen by dental lab technicians.

The effect on the final product is primarily a significant reduction of the reaction layer levels.

As this also eliminates the requirement of a hydrofluoric acid etching treatment, the product will also have the secondary effects of having a higher tensile strength and better surface quality. Deterioration of both the latter effects would normally be seen as a result of hydrofluoric acid etching as described in the cited documents “Braz Dent J (2011) 22(1) and J Prosthodont. 2008 Jul;17(5):415-9. Epub 2008 May 9”.

The aforementioned investment binder forms a surface reaction controlled interface layer between a base material, preferably a ceramic material, and a glass material and/or metal alloy and/or a precious metal. In this manner, the formation of an aforementioned reaction layer can be prevented. This results in a cleaner formed glass ceramic tooth, which needs less polishing with much less aggressive and more environmentally sensitive cleaning agents. Therefore there is no use of hydrofluoric acid for the polishing compared to the state of the art.

An aforementioned investment binder can be combined with a matrix of investment selected by a customer. The investment binder reacts with the metal, metal alloy or glass being formed on the moulding structure, forming an artificial tooth.

The investment binder can be made by the following formulation:

-   -   An investment formulation consisting of but not limited to:     -   10% Millisil W10 (quartz silica powder supplied by Quarzwerke,         Frechen)     -   50% Millisil W12     -   8% Sikron SF8000 (cristobalite silica powder supplied by         Quarzwerke, Frechen)     -   7% Sikron SF6000 (cristobalite silica powder supplied by         (Quarzwerke, Frechen)     -   25% CerSett DB30 (a proprietary binder powder from Cermatco Ltd,         Aylesham comprising a CerMag FC-Slow (a magnesium oxide powder))         and CermaDent DB30 (a stabilised mono ammonium phosphate powder         co-milled with a surface reaction control agent in this case         boron nitride powder of d50=3.8 micron together with added         aluminium oxide powder)

25% (24 mls/100 g) of this investment formulation and 75% diluted silica sol liquid 300/30 supplied by H. C. Starck are mixed under vacuum for 3 minutes. The slurry is then poured into a silicone 100 g muffle around a second premolar crown made with wax type Yeti Grey Ashless and is allowed to set for 30 minutes at 21 deg C. A speed heat technique at 850 deg C. is used for 30 minutes without cracking A lithium disilicate ingot of type E-max LT D4 (Vivadent-Ivoclar) is then hot pressed at 850 deg C. and 4.5 bar pressure.

The glass ceramic molar can be removed and cleaned using 2.0-2.5 bar of pressure with 50 micron glass beads alone and without use of hydrofluoric acid.

All %-indications are percentages by weight (w/v). 

1-6. (canceled)
 7. An investment binder for forming a binder phase when applied to an investment casting, wherein the investment binder comprises I at least a first compound as a source of magnesium-ions, II a second compound as a source of phosphate-ions and III a third compound being a surface reaction control agent incorporated in the first or the second compound.
 8. The investment binder according to claim 7, wherein the surface reaction control agent is selected from the group consisting of hexagonal boron nitride, ion-exchange agents, zeolites and clay-containing minerals, including bentonites.
 9. The investment binder according to claim 7, wherein the surface reaction control agent is provided for a surface treatment.
 10. The investment binder according to claim 7, wherein the second compound is an alkali metal-phosphate or an ammonium-phosphate.
 11. Use of the investment binder according to claim 7 for the formation of a dental investment casting, wherein the investment binder forms a surface reaction controlled interface layer between a base material, preferably a ceramic material, and a glass material and/or metal alloy and/or a precious metal.
 12. Use of the investment binder according to claim 11, wherein the investment binder is used for preventing or controlling the formation of reaction layers between said base material and said glass material and/or metal alloy and/or precious metal when applying the said glass material and/or metal alloy and/or precious metal to the base material. 